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IC 9144 



Bureau of Mines Information Circular/1987 



Spontaneous Combustion Fire Detection 
for Deep Metal Mines 



By William H. Pomroy 




UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 9144 



Spontaneous Combustion Fire Detection 
for Deep Metal Mines 



By William H. Pomroy 




UNITED STATES DEPARTMENT OF THE INTERIOR 
Donald Paul Hodel, Secretary 

BUREAU OF MINES 
Robert C. Horton, Director 




Library of Congress Cataloging in Publication Data: 






Pomroy, William H. 

Spontaneous combustion fire detection for deep metal mines. 

(Information circular; 9144) 

Supt. of Docs, no.: I 28.27: 9144. 

1. Mine fires. 2. Combustion. Spontaneous. 3. Fire detectors. 4. Metal sulphides. 5. 
Mines and mineral resources. I. Title. II. Series: Information circular (United States. 
Bureau of Mines); 9144. 



^TN295JJ4- * [TN315] 622 s [622'.8] 86-600294 



CONTENTS 



Page 



Abstract 1 

Introduction • 

First-generation spontaneous combustion fire detection system 2 

Characterization of spontaneous combustion fires in metal mines 3 

Identification of composition of spontaneous combustion fuel 3 

Identification and quantification of products of combustion resulting from 

spontaneous combustion 4 

Organic vapor evolution 6 

Analysis of test results * 6 

System design 7 

Laboratory tests 10 

In-mine test 11 

Second-generation spontaneous combustion fire detection system 16 

Design modifications 17 

Laboratory tests 19 

In-mine tests 19 

Summary and conclusions 25 

ILLUSTRATIONS 

1. Determination of fire hazard of various compositions of spontaneous 

combustion fuel 4 

2. Combustion products for sawdust mixtures 5 

3. Surface telemetry and recorder assembly 11 

4. Laboratory testing of spontaneous combustion fire detection system 12 

5. Results of laboratory testing 12 

6. Immediate vicinity of in-mine test 13 

7 . Mine level on which in-mine test was conducted 13 

8. Fire detection enclosures 14 

9. Induced spontaneous combustion test fire underground 16 

10. Results of in-mine induced spontaneous combustion testing 17 

11. Surface telemetry and control unit for second-generation spontaneous 

combustion system 18 

12. Layout of major components of second-generation spontaneous combustion 

fire detection system 19 

13. Section through test mine showing locations of sensor assemblies 20 

14. Sensor assembly B, prewired and mounted on panel 21 

15. Layout for in-mine fire test of second-generation spontaneous combustion 

fire detection system 22 

16. Igniting test fire 23 

17. Test fire burning 23 

18. CO and C0 2 measured during fire test 23 

19. Smoke measured during fire test 24 

TABLES 

1. Fuel compositions and chamber temperature set points for tests to identify 

and quantify products of combustion resulting from spontaneous combustion 6 

2. Comparison of fire detection systems using pneumatic tube bundle and 

electronic telemetry 8 

3. Comparison of CO and C0 2 concentrations measured by detection system 

versus laboratory analysis 24 





UNIT OF MEASURE 


ABBREVIATIONS USED IN THIS REPORT 


°F 


degree Fahrenheit 




mA 


milliampere 


ft 


foot 






mCi 


raillicurie 


ft 3 


cubic foot 






min 


minute 


ft 3 /min 


cubic foot per 


minute 


oz 


ounce 


gal 


gallon 






pet 


percent 


h 


hour 






pct/ft 


percent per foot 


Hz 


hertz 






ppm 


part per million 


in 


inch 






s 


second 


in/h 


inch per hour 






scf/h 


standard cubic foot per hour 


kHz 


kilohertz 






V 


volt 


lb 


pound 






vol/h 


volume per hour 


lb/ft 3 


pound per cubic 


: foot 


yr 


year 



SPONTANEOUS COMBUSTION FIRE DETECTION 
FOR DEEP METAL MINES 



By William H. Pomroy 1 



ABSTRACT 

Spontaneous combustion fires involving high-sulf ide-content ores are a 
relatively infrequent yet serious safety hazard in mining. They are 
also the cause of lengthy mine shutdowns because they typically occur in 
areas of a mine where abundant fuel material is present but which are 
inaccessible for fire fighting. This Bureau of Mines report describes 
research to design, fabricate, and test in the laboratory and field a 
system that warns of spontaneous combustion fires in metal mines. Over- 
all performance of the detection system was found to be satisfactory, in 
that the system was capable of reliably detecting low levels of com- 
bustion products believed to indicate the preflaming stage of sponta- 
neous combustion in metal mines. Installation of similar systems in 
mines with a high risk of spontaneous combustion is recommended. Prin- 
cipal operating problems and recommended corrective actions are also 
discussed. 

Group supervisor, Twin Cities Research Center, Bureau of Mines, Minneapolis, MN. 



INTRODUCTION 



Exothermic oxidation reactions in metal 
sulfide ores can occur in underground 
mines. If the heat generated by these 
reactions is not dissipated, temperatures 
sufficient for rapid oxidation and com- 
bustion of both the sulfides and the 
adjacent timber and other mine combusti- 
bles may be produced. Although spontane- 
ous combustion fires are relatively in- 
frequent, accounting for only about 2 pet 
of all underground noncoal mine fires, 
they are generally quite disruptive to 
mine operations and represent a signifi- 
cant safety hazard to miners. Spontane- 
ous combustion fires often start in 
abandoned, backfilled, and/or caved mine 
areas where access for fire-fighting 
operations is difficult or impossible. 
Compounding the problem of accessibility 
is the large amount of fuel that is gen- 
erally available to a spontaneous com- 
bustion fire. Fires on discrete pieces 
of equipment are generally of short dura- 
tion because they self-extinguish when 
the available fuel is consumed. However, 
the large quantity of support timber 
present in many older mines can provide 
fuel sufficient for fires of many months' 
duration, and spontaneous combustion 
fires typically involve such support tim- 
ber. Since 1950, about 57 pet of noncoal 
underground mine fires lasting longer 
than 24 h were caused by spontaneous 
combustion. 

Though research is now under way, the 
precise chemical and physical mechanisms 
giving rise to sulfide oxidation and 
spontaneous combustion in mines are 
still not well understood. Hence, pre- 
vention of spontaneous combution fires is 



generally limited to sealing abandoned, 
backfilled, and caved areas known to be 
susceptible to self -heating, in the hope 
that the denial of oxygen will retard the 
oxidation processes. 

Sealing is, however, an imperfect solu- 
tion, and despite efforts to locate and 
seal "hot spots," high-risk mines are 
still vulnerable to spontaneous combus- 
tion events. Oxygen can leak into sealed 
areas as a result of ground movement, 
caving, natural fracturing, and faulty 
bulkhead construction. And spontaneous 
combustion fires can occur outside sealed 
areas. As an added precaution to fire 
prevention efforts, systematic fire de- 
tection is practiced at most high-risk 
mines. Fire detection can include atmos- 
phere monitoring behind bulkheads, as 
well as temperature and air quality 
checks in working areas. 

In 1978, the Bureau of Mines embarked 
on a research program to upgrade the 
technology for detecting spontaneous com- 
bustion fires, to fabricate a prototype 
system for spontaneous combustion fire 
detection, and to perform in-mine valida- 
tion tests of this system. The work was 
performed under contract; preliminary re- 
search findings, covering the development 
and testing of a first-generation sponta- 
neous combustion fire detection system, 
are contained in the contractor's final 
report. 2 

In addition to summarizing those find- 
ings, this Bureau report describes the 
design and long-term, in-mine testing of 
an improved, second-generation spontane- 
ous combustion fire detection system. 



FIRST-GENERATION SPONTANEOUS COMBUSTION FIRE DETECTION SYSTEM 



Development of a first-generation spon- 
taneous combustion fire detection sys- 
tem was accomplished in four steps: 
(1) characterization of spontaneous com- 
bustion fires in metal mines, (2) design 



of a spontaneous combustion fire detec- 
tion system, (3) laboratory testing, and 
(4) field testing in an operating under- 
ground mine. 



^Stevens, R. B. Improved Spontane- 
ous Combustion Protection for Under- 
ground Metal Mines (contract HO282002, 



FMC Corp.). BuMines OFR 79-80, 1979, 
262 pp.; NTIS PB 80-210461. 



CHARACTERIZATION OF SPONTANEOUS 
COMBUSTION FIRES IN METAL MINES 

Accelerated spontaneous combustion 
tests were performed in the laboratory to 
identify and quantify the products of 
combustion that could be expected to re- 
sult from an actual spontaneous combus- 
tion event in a mine. These data were 
then used to guide the selection of de- 
tection instruments to be incorporated 
into the prototype system for spontaneous 
combustion fire detection. 

Several test series were conducted. 
Significant results are summarized in the 
following sections. 

Identification of Composition 
of Spontaneous Combustion Fuel 

Samples consisting of sawdust and saw- 
dust with sulfide and sulfur additives 
were evaluated in a specially designed 
chamber under mine conditions (95° F, 95 
pet relative humidity (RH)) and induced 
spontaneous combustion conditions (300° 
to 400° F) to determine which fuel com- 
positions represent the most serious 
spontaneous combustion hazard. The fuel 
compositions shown to exhibit the great- 
est propensity toward self-heating were 
used in follow-on tests to identify and 
quantify the products of combustion in- 
dicative of spontaneous combustion. 

An 8-ft '-capacity chamber with variable 
temperature capability from ambient to 
400° F and variable RH capability from 20 
to 95 pet was used. The chamber was 
equipped for sample temperature monitor- 
ing, variable chamber ventilation, and 
gas sampling. The ventilation flow rates 
were 0, 2, 4, and 8 scf/h (correspond- 
ing to 0-, 0.25-, 0.5-, and 1.0-vol/h 
exchange rates in the chamber). In the 
initial test sequence, eight approximate- 
ly 1-ft 3 samples were tested. The eight 
samples included 10- and 20-lb/ft 3 pack- 
ing densities of the following composi- 
tions: pure sawdust, sawdust plus 5 pet 
FeS, sawdust plus 1 pet S, and sawdust 
plus 5 pet FeS and 1 pet S. These compo- 
sitions corresponded to samples collected 
at several underground metal mines and 



were believed to represent medium- to 
high-risk, spontaneous combustion fuel ma- 
terials. After 144 h at 95° F, 95 pet 
RH, chamber conditions were altered to 
120° F, 95 pet RH, and held for 186 h. 
Finally, the temperature was increased to 
145° F, 95 pet RH, and held for an addi- 
tional 144 h (for a total of 19 days, 18 
h). Under these conditions, none of the 
samples showed signs of spontaneous heat- 
ing. Five of the samples, all containing 
FeS and/or sulfur, were then heated to 
higher temperatures (180°, 210°, 250°, 
300° F) in an effort to induce spontane- 
ous combustion under conditions that 
might be encountered if combustibles were 
located near a self-heating ore body. 
Although some decomposition of the sam- 
ples occurred under these conditions, 
producing high CO and CO2 levels, no evi- 
dence of self-heating was observed. At 
338° F, all samples underwent combustion. 
In similar tests where sawdust with 5 pet 
FeS and 1 pet S was aged for 123 h at 
95° F, 95 pet RH, spontaneous combustion 
was Induced by elevating the chamber tem- 
perature to as low as 293° F. However, 
when sawdust alone (no FeS or sulfur ad- 
ditives) was pretreated at elevated tem- 
peratures and humidities for extended pe- 
riods, no self-heating was observed, even 
at chamber temperatures of 350° F. The 
appearance of the residue indicated that 
wood modified by long-term treatment at 
elevated temperature and humidity in 
the presence of FeS and/or sulfur burned 
more completely than unmodified wood. In 
addition, the combustion temperatures of 
the aged, FeS- and/or sulfur-treated sam- 
ples, ranging from 295° to 340° F, are 
considerably lower than the literature 
value of 392° F for untreated wood, indi- 
cating reduced ignition points for woods 
in the presence of FeS and/or sulfur 
contamination. 

Based on these test results , further 
experiments were conducted in the chamber 
to delineate the Ignition potential of 
five high-risk, fuel compositions when 
heated for prolonged periods at higher 
pretreatment temperatures: (1) aged saw- 
dust plus 5 pet FeS and 1 pet S, (2) aged 
sawdust plus 5 pet FeS, (3) aged sawdust 



plus 1 pet rotted wood, (4) aged sawdust 
with no additives, and (5) virgin saw- 
dust. After 118 h at 250° F, the chamber 
was shut off for thermocouple repairs. 
The chamber was restarted at 302° F. 
Both samples containing FeS and sulfur 
then began to display visible signs of 
smoldering, including smoke generation 
and pile discoloration. However, the 
three samples containing virgin and aged 
sawdust and rotted wood, but no FeS or 
sulfur additives, did not undergo combus- 
tion. These results, summarized in fig- 
ure 1, confirm results of earlier tests 
at lower pretreatment temperatures. 

These brief laboratory studies cannot 
absolutely rule out the possibility that 
wood alone may be responsible for sponta- 
neous combustion in mines. However, the 
evidence strongly suggests that mine tim- 
bers alone do not exhibit self -heating 
tendencies when exposed to conditions 
that prevail in most underground metal 
mines. Rather, the principal source of 
spontaneous combustion appears to be 



425 



400 



375 



350 - 



325 - 



300 



275 - 



250 



1 1 1 1 1 1 

KEY 




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Aged sawdust +5 pel FeS *l pel S 






Aged sawdust +5 pet FeS 






Virgin sawdust; aged sawdust (no additives); 






aged sawdust (1 pet rotted wood) 






Oven set temperature 




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118 119 120 121 122 123 124 125 126 

HOURS INTO TEST 

FIGURE 1.— Determination of fire hazard of various com- 
positions of spontaneous combustion fuel. 



high-sulf ide-content ores that ignite 
secondary combustibles, especially timber 
modified by long-term exposure to high 
temperatures and humidities, and contami- 
nated by sulfides and/or sulfur. 

Identification and Quantification 

of Products of Combustion Resulting 

From Spontaneous Combustion 

Twenty-eight fixed-temperature pyro- 
lytic decomposition tests of several 
high-risk compositions of spontaneous 
combustion fuel were performed in a 128- 
ft -capacity chamber. The objective was 
to identify and quantify the mix of com- 
bustion products that evolve during the 
preflaming stage of a spontaneous combus- 
tion fire. This stage may last from 1 
day or less to several weeks or more. 
The goal of spontaneous combustion fire 
detection is to detect this characteris- 
tic mix of combustion products during the 
preflaming stage, so that appropriate 
emergency procedures (evacuation, fire 
fighting, etc.) can be initiated before 
the fire reaches the flaming combustion 
stage and threatens rapid growth and con- 
taminant spread. The chamber was instru- 
mented for monitoring smoke obscuration, 
CO2, CO, hydrocarbons, oxygen, and SO 2. 
The samples were contained in a 6.3-in- 
diam by 3.0-in-high cylindrical pan. 
Eight thermocouples were arrayed in the 
sample to measure sample temperatures 
throughout each test. The compositions 
of fuels and chamber temperature set 
points tested are shown in table 1. Heat 
was applied to the samples through the 
chamber floor. All tests were limited to 
60 min. 

Figure 2 depicts the most significant 
results of this test series. Sawdust 
plus 1 pet rotted wood (rotted wood col- 
lected from several sulfide mines) heated 
to 410° F (fig. 24) produced moderate 
levels of CO and CO2 after 50 min. The 
5-ppm-S02 level is believed to result 
from long-term sulfide exposure of the 
rotted wood samples in the mine. Sawdust 
plus 5 pet FeS heated at 410° F (fig. 25) 
produced high levels of CO and CO2 after 
45 min, as well as moderate levels of 
SO2, smoke, and oxygen depletion after 



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30 40 

TIME, min 



FIGURE 2.— Combustion products for sawdust 
mixtures. A, Sawdust plus 1 pet rotted wood heated 
to 41 ° F; B, sawdust plus 5 pet FeS heated to 41 ° 
F; C, sawdust plus 1 pet S and 5 pet FeS heated to 
410° F; D, sawdust plus 5 pet CuS heated to 410° F; 
£, sawdust plus 5 pet ZnS heated to 410° F; F, 
sawdust plus 1 pet S and 5 pet FeS heated to 520° 
F; G, sawdust plus 1 pet S and 5 pet FeS heated to 
800° F. 



20 30 40 

TIME, mihr 



50 60 



TABLE 1. - Fuel compositions and chamber temperature set points 
for tests to identify and quantify products of combustion 
resulting from spontaneous combustion 

(X indicates testing) 



Fuel compositions tested 


390° F 


410° F 


465° F 


520° F 


590° F 


800° F 


Virgin sawdust: 


X 
X 


X 
X 
X 
X 
X 
X 


X 
X 
X 

X 
X 
X 


X 
X 
X 
X 

X 
X 


X 
X 
X 
X 

X 


x 


1 pet S, 5 pet FeS.... 


X 
X 
X 










5 Dct PbS 




Rotted wood (100 pet)... 





50 min. Sawdust plus 1 pet S and 5 pet 
FeS heated at 410° F (fig. 2c ) produced 
very high levels of S0 2 and oxygen de- 
pletion after 30 min. CO and C0 2 also 
reached very high levels after 35 min; 
however, smoke levels were only moderate 
after 60 min. 

The samples of sawdust plus 5 pet CuS 
(fig. 2D) and sawdust plus 5 pet ZnS 
(fig. IE) heated at 410° F followed much 
the same pattern as the sawdust plus 5 
pet FeS, producing high levels of CO and 
C0 2 after 45 min and moderate levels of 
S0 2 , smoke, and oxygen depletion after 
50 min. At higher temperatures (figs. 
2F-2G) , evolution of combustion pro- 
ducts, especial-ly S0 2 , occurred more 
rapidly (closed-cup flashpoint of sulfur 
is 405° F, autoignition temperature is 
450° F). 

Organic Vapor Evolution 



The presence of a sweet odor, or "sweet 



gas. 



is frequently reported by miners 



prior to outbreaks of spontaneous combus- 
tion fires. This sweet gas is believed 
to be a mix of organic vapors produced 
during the preflaming stage of a sponta- 
neous combustion fire. Thus, organic va- 
por detection was considered for possible 
inclusion in the prototype spontaneous 
combustion fire detection system. Dif- 
ferential scanning calorimetry evolved- 
gas analysis was performed in order to 
determine whether the quantity of organic 



vapor production would be sufficient for 
detection. 

Methane, propane, formaldehyde, metha- 
nol, acetaldehyde, and acetone were the 
primary species detected. Trace levels 
were measured at temperatures as low as 
212° F, with moderate levels observed in 
most species between 392° and 482° F. 
Peak concentrations occurred in all spe- 
cies at 662° F. 

Analysis of Test Results 

Of the four gases examined in detail in 
the pyrolytic decomposition studies, oxy- 
gen and CO respond sooner and at higher 
levels than C0 2 and S0 2 . However, the 
high levels of CO and C0 2 measured dur- 
ing the low-temperature (302° to 662° F) 
studies in the 8-ft 3 chamber suggest the 
value of both of these gases as early in- 
dicators. In addition, S0 2 detection may 
warn of a sulfide heating event that does 
not take place in the presence of second- 
ary combustibles. The failure of smoke 
obscuration levels to exceed the standard 
2 pct/ft in the majority of the pyrolytic 
decomposition tests (below 482° F, the 
levels ranged from 0.5 to 2.0 pct/ft) im- 
plies that smoke detection may not offer 
an advantage over gas detection. Organic 
vapors, though produced in sufficient 
quantities at higher temperatures, were 
not proven reliable indicators at temper- 
atures below 392° F. 



Although air temperature sensing was 
not specifically addressed in these 
studies, it is practiced at most mines 
with recognized spontaneous combustion 
problems. A temperature rise may be the 
first indication of a self -heating event, 
especially if secondary combustibles 
are not immediately adjacent to the hot 
spot. 

Thus, the recommended mix of detection 
instruments to provide early warning of a 
spontaneous combustion fire in an under- 
ground metal mine includes those for oxy- 
gen, SO2, CO, CO2, and temperature. The 
recommended sampling ranges of the in- 
struments, as indicated by the test pro- 
gram, are as follows: O2, to 21 pet; 
S0 2 , to 30,000 ppm; CO, to 50 ppm (0 
to 500 ppm if sampling behind bulkhead); 
CO2, to 30,000 ppm; and temperature, 
0° to 150° F. 

SYSTEM DESIGN 

Two basic detection system configura- 
tions were considered: a pneumatic "tube 
bundle" approach and a fully electronic 
telemetry approach. Pneumatic tube bun- 
dle fire detection involves sampling the 
mine atmosphere at various locations 
through a network of plastic tubes to 
which a vacuum is applied. The airflows 
from each tube are sequentially directed 
to a central analytic station equipped 
for gas monitoring. The fully electronic 
telemetry approach involves the placement 
of detection instruments at each under- 
ground site to be monitored. Detector 
outputs are transmitted to a central con- 
trol point over electronic telemetry 
lines. Some advantages and disadvantages 
of each approach are outlined in table 2. 

The selection of a fully electronic te- 
lemetry configuration for the prototype 
spontaneous combustion fire detection 
system was based on two factors: First, 
only one sampling point was anticipated 
for this experimental system; thus, cost 
tended to favor the telemetry approach. 
This cost advantage could be magnified 
greatly if existing telemetry lines at 
the in-mine test site could be used (this 
indeed was the case at the test mine). 



Second, air temperature was identified as 
an important parameter to be measured, 
and air temperature cannot be detected by 
tube bundles. 

The selection of detection instruments 
was based on their field-proven resist- 
ance to harsh environmental exposures 
(heat, cold, humidity, dust, blast fumes, 
diesel exhaust, input voltage fluctua- 
tions, transients, and electromagnetic 
interference). 

The oxygen detector was a diffusion 
electrochemical cell type. The microfuel 
cell consumes oxygen from the atmosphere 
and generates a current proportional to 
the concentration. Life of the cell is 
approximately 12 months at 25 pet O2. No 
routine maintenance is required except 
periodic recalibration. Since use under 
the environmental exposures anticipated 
underground (heat and humidity primarily) 
was nonstandard, a determination of re- 
calibration frequency would be made as 
part of the in-mine test. 

SO2 detection was provided by a nondis- 
persive infrared absorption-type detec- 
tor. The unit, including the analyzer 
cell and power supply, are housed in a 
rugged fiberglass enclosure designed for 
underground installation. A dust filter 
is fitted to the sample plenum. No pumps 
or other auxiliary sample delivery or 
pretreatment are required, and the unit 
contains no moving parts. 

The analyzer detects the attenuation of 
radiation due to molecular absorption by 
the sample gas. A Nichrome^ filament 
pulsed at a specified frequency radiates 
broadband energy. This energy passes 
through the sample gas in a reflective 
optical chamber, through a spectral fil- 
ter, and is measured by a pyroelectric 
cell photo detector. The electrical sig- 
nal output is inversely proportional to 
the gas concentration and varies loga- 
rithmically with concentration. Selec- 
tivity to the sample gas is determined by 
the band-pass spectral filter. The de- 
tector's output signal does not indicate 

■^Reference to specific products does 
not imply endorsement by the Bureau of 
Mines. 



TABLE 2. - Comparison of fire detection systems using pneumatic tube bundle 
and electronic telemetry 



Pneumatic tube bundle 



Electronic telemetry 



MAINTENANCE 



Simple for analyzers, pumps, controls, 
etc. , because all electronics and 
mechanical components can be located on 
surface. Plugs or breaks in sample 
tubes can be difficult to locate and 
repair. 



Access to some underground locations, for 
maintenance, calibration, and/or repair 
of equipment can be a problem. Trouble- 
shooting telemetry lines (opens, faults, 
etc. ) is generally easier than trouble- 
shooting sample tubes (plugs breaks). 



AREA OF COVERAGE 



In large, spread-out mines, the time re- 
quired for a sample to travel the entire 
length of a sample tube can be a limit- 
ing factor. E.g. , a 3/8-in sample tube 
10,000 ft in length will result in a 
tube travel time of about 17 min. 



Very long transmission distances (5 to 15 
miles) may require special telemetry 
provisions. 



COST 



In general, the larger the system (i.e., 
number of sampling points), the lower 
the cost per sampling point because only 
one analytic station is required. Like- 
wise, however, small systems have a rel- 
atively high cost per sample point. 



Since each sampling point requires a full 
compliment of detection instruments, 
system costs increase in proportion to 
the number of sampling points. Teleme- 
try line acquisition and installation 
costs are generally lower than for tube 
bundles servicing the same number of 
sampling points, especially if multiplex 
data transmission is employed. 



MEASUREMENT PRECISION AND RELIABILITY 



Because the analytic station can be on 
surface in a relatively clean environ- 
ment, high precision detection instru- 
ments can be utilized. Also, in that 
location, detector maintenance and 
calibration is likely to be frequent. 



Measurement precision and reliability is 
directly proportional to the adequacy of 
detector maintenance and calibration. 
Where accessibility is difficult, mea- 
surement precision and reliability may 
be low. 



NEED FOR ELECTRIC POWER 



Since all electronic equipment can be on 
surface, electric power is not required 
at each sampling point — a definite ad- 
vantage if underground power is lost 
during a mine emergency. 



In general, electric power is required at 
each sampling point; however, some 
detectors can be powered through the 
telemetry lines, (i.e., from surface). 



TABLE 2. - Comparison of fire detection systems using pneumatic tube bundle 
and electronic telemetry — Continued 



Pneumatic tube bundle 



Electronic telemetry 



PRODUCTS OF COMBUSTION THAT CAN BE DETECTED 



Most gases, including oxygen, CO, and 
C0 2 , can be accurately measured. Smoke 
particles tend to diffuse into the tube 
walls; thus, depending on the tube 
length, diameter, sample air velocity 
and other factors, smoke detection may 
or may not be possible for a given 
application. Some gases, such as N0 X 
and S0 2 , react with the tube or the wa- 
ter that may collect in the tubes, 
producing erroneous readings. Measure- 
ments of air velocity, direction, and 
temperature at the sampling point are 
impossible. 



Any gas, particulate, or condition (air 
velocity, direction, temperature, etc.) 
for which a detection instrument exists 
can be measured. 



ENVIRONMENTAL EXPOSURE FACTORS 



Analytic station can be located in a 
clean environment. Tubes are subject to 
plugging from dust accumulation and 
condensation of humid sample air. 
Freezing conditions further exacerbate 
humidity and condensation problems. 
Broken lines may result from abrasion, 
cuts, roof falls, etc. 



Ruggedness and resistance to harsh envi- 
ronmental effects (heat, humidity, cold, 
dust, diesel exhaust, blast fumes, etc.) 
are essential for detection instruments. 
Both detection instruments and telemetry 
lines may be affected by electromagnetic 
interference from nearby power lines and 
other sources. Broken telemetry lines 
may result from roof falls, etc.; how- 
ever, redundant telemetry lines, telem- 
etry loop configurations, etc., can be 
employed to improve telemetry reliabil- 
ity. Input voltage fluctuations and 
electrical transients (voltage spikes) 
can also be expected underground. 



10 



accurate gas concentrations without ref- 
erence to calibration curves, but normal 
gas levels are easily distinguishable 
from excursions that could result from a 
spontaneous heating event. Input power 
is 110 V ac, 50 to 60 Hz; output is to 
1 V dc analog; and the range is to 
30,000 ppm S0 2 . 

An identical infrared analyzer, fitted 
with a different spectral filter, was 
used for CO2 detection. The output range 
of the CO2 analyzer is also to 30,000 
ppm. 

The CO detector selected was an elec- 
trochemical fuel cell type developed for 
underground mine use. The unit is housed 
in a rugged fiberglass enclosure. A 
mechanical pump is provided to draw sam- 
ple gas into the cell. CO is oxidized to 
CO2 at a potentially controlled elec- 
trode, and the current produced is pro- 
portional to the partial pressure of CO 
in the atmosphere. An electronic circuit 
then amplifies the signal to the 0- to 
1-V-dc output level. The unit's output 
range is to 50 ppm CO; however, the de- 
tector can be retrofitted with new cells 
at nominal cost to alter the range (0 to 
10, to 500, to 2,000 ppm, etc.). The 
life of the cell may vary from 6 to 12 
months; however, monthly zero and span 
checks are recommended. Past experience 
also indicated that the sample pump re- 
quired occasional checks. 

Temperature detection was provided by 
an electrical resistance-type sensor. A 
1-mA current is passed through an elec- 
trical circuit whose resistance varies 
with temperature. The signal is lin- 
earized and amplified, yielding a 0- to 
1-V-dc output proportional to the 0° to 
150° F range of the instrument. 

Selection of a telemetry system was 
based on several factors, including 
proven resistance to harsh environmental 
exposures; compatibility with existing 
mine telephone lines, chart recorders, 
and detection instruments; capability to 
transmit both analog and digital signals 
over distances up to 30,000 ft; minimum 
capacity of six outstations (each with 
five detection instruments); low cost; 
and minimal maintenance requirements. 



Fifteen candidate systems were evaluated 
with respect to these selection criteria. 

The system chosen is manufactured in 
the Republic of South Africa and is used 
primarily in deep South African gold and 
coal mines for fire detection. It em- 
ploys low-frequency division multiplex 
telemetry to transmit up to 48 signals 
over 1 twisted-wire pair. The system 
spans a frequency band of 0.3 to 10 kHz. 
Telephone conversation over the twisted 
pair does not interfere with data trans- 
mission. Operating on a balanced-line 
principle and incorporating special line 
filters and protection networks, the sys- 
tem is designed to be noise immune and 
interference free. Both the transmitters 
(underground) and receivers (surface) are 
housed in rugged fiberglass enclosures 
and are intended for mine installation. 

Since the focus of this research pro- 
gram was detection system design and 
function, the cost of an elaborate system 
control and alarm master panel could not 
be justified. A simple means for record- 
ing long-terra trends in the sensed param- 
eters and distinguishing shorter terra 
excursions from these baseline values 
(which might result from a spontaneous 
heating event) was sufficient. To sim- 
plify output display and data recording, 
a multipoint chart recorder was selected. 
The recorder uses colored ink ribbons and 
a "chopper bar" printer to display six 
channels of data. A chart speed of 1.25 
in/h was selected to provide 30 days be- 
tween paper changes. Data are recorded 
at approximately 2-s intervals. Figure 3 
shows the surface telemetry and recorder 
assembly. 

LABORATORY TESTS 

The spontaneous combustion fire warn- 
ing system was laboratory tested to ver- 
ify the compatibility of the various 
system elements; to identify potential 
in-mine installation, operation, mainte- 
nance, and troubleshooting problems; and 
to evaluate system performance under 
controlled conditions. Laboratory test- 
ing was focused in four areas: (1) long- 
terra calibration, zero drift, and 



11 




FIGURE 3.— Surface telemetry and recorder assembly. 



stability, (2) telemetry system perform- 
ance, (3) effects of harsh environmental 
exposures, and (4) observation and docu- 
mentation of induced system malfunctions. 
Simulated conditions included power-line 
voltage fluctuations, long-terra black- 
outs, high and low temperature and humid- 
ity extremes, and electromagnetic noise 
interference. The system was finally 
subjected to an induced spontaneous com- 
bustion fire to assess overall response 
to abnormal contaminant levels. Figure 4 
represents the physical layout of the in- 
duced spontaneous combustion testing. 

After a "burn-in" period sufficient to 
achieve stable readings from the detec- 
tors, the system was functionally tested 
by exposure to combustion products from 
an induced spontaneous combustion event 
in the 128-ft 3 chamber. A fuel sample, 
consisting of sawdust plus 5 pet FeS and 
1 pet S treated for 2 weeks at high tem- 
perature and humidity, was heated to ap- 
proximately 480° F. Figure 5 is a re- 
cording of the detector outputs. 



All system elements functioned proper- 
ly, with good agreement between the de- 
tection system and the laboratory analyz- 
ers. A peak CO2 level of 1,400 ppm was 
recorded 8 min after the heat source was 
removed from the sample. A peak SO 2 lev- 
el of 60 ppm occurred 5 min after the 
heat source was removed. CO levels in 
excess of the 50-ppm range of the CO de- 
tector were recorded within 12 min of the 
start of the test. The oxygen content of 
the atmosphere inside the chamber dropped 
to a low of 19 pet at 8 min into the 
test. The temperature peak occurred at 
35 min. 

IN-MINE TEST 

Selection of a site for in-mine testing 
was based on four criteria: (1) mine 
fire history, spontaneous combustion oc- 
currences, and/or known heating condi- 
tions, (2) a high level of management in- 
terest and commitment to support system 
installation, operation, and maintenance, 



12 



Laboratory analyzers 



Spontaneous combustion fire detection system 



CO, 



2£ 



Temp 



Pan Opt 
tempi 



IT 



/ \ 



HHSELJ 



Receiver 



Surface receiver and \ 

recorder assembly Recorder - 



Underground 

telemetry 

assembly 



l 



' Optical path 



C= 




-Thermocouple and 
anemometer 




— — Exhaust 



('0 OTJT \ 



-Fuel 
pan 



Test chamber 



FIGURE 4.— Laboratory testing of spontaneous combustion 
fire detection system. 



(3) ease of access for project personnel, 
and (4) the availability of power and 
dedicated telemetry lines in the area 
designated for equipment installation. 
Three mines satisfying all the selection 
criteria were identified. 

Unfavorable economic conditions, howev- 
er, resulted in a cessation of operations 
at two of the candidate mines. An agree- 
ment was negotiated with the third mine, 
a deep, hot underground copper mine in 
Arizona, to permit the required tests. 
The mine operated three shifts per day 
using the underhand cut-and-fill mining 
method. 

Discussions with mine personnel led to 
the decision to install the detection 
system at a point 3,600 ft underground 




CO, ppm 
C0 2 , ppm 
S0 2 , ppm 
2 , pel 
Temp, "F 



15 20 25 30 35 40 45 50 





25 





10,000 


1,700 


760 


290 




10,000 


1,700 


760 


290 


3 


5 


10 


15 


20 



30 45 60 75 90 105 120 135 150 

FIGURE 5.— Results of laboratory testing. 

and approximately 3,100 ft horizontally 
from the shaft used for personnel and 
supply access. The system was installed 
in an inactive shop carrying exhaust air 
from several mine production and worked- 
out areas. Figures 6 and 7 depict the 
immediate vicinity of the testsite and 
the mine level on which it was located. 
Airflow through the area was approxi- 
mately 20,000 ft 3 /min, and temperature 
and relative humidity were both in the 
mid-90' s. 

The detection instruments and under- 
ground telemetry modules were installed 
on mounting panels, prewired, and en- 
closed in fiberglass housings for ease 
of transport and installation (fig. 8). 
Mine installation and an initial operat- 
ing checkout were accomplished over a 
1-week period. Telephone communication 
was maintained between the underground 
testsite and the surface telemetry and 
recorder site during zero purge and span 



13 



■Underground telemetry assembly 
und sensor assemt 




FIGURE 6.— Immediate vicinity of in-mine test. 



N6000 




N4000 



Main shaft (downcast) 
9 shaft 



400 



Scale, ft 



400 



FIGURE 7.— Mine level on which in-mine test was conducted. 



i4 




FIGURE 8.— Fire detection enclosures. 



calibration of the sensors to assure that 
all signals were aligned and functioning 
properly. 

Performance of the detection instru- 
ments over the 5-month in-mine test was 
generally good except for early problems 
with the CO and oxygen detectors. The CO 
detector experienced sample pump and dc 
circuit card failures during the first 
month of operation. The pump was re- 
placed, and the circuit card failure was 
traced to a grounding incompatibility 
between the negatively grounded battery 
backup power supply on the detector and 



the positively grounded telemetry system. 
The detector was operated on ac power for 
the remainder of the test period, and no 
further detector failures occurred. 

While the CO sensor was under repair, 
a second CO sensor was installed as a 
temporary replacement. This replacement 
unit was a catalytic semiconductor type 
that responds preferentially to CO but is 
also sensitive to other oxidizable gases. 
When the original CO sensor was returned, 
both units were operated simultaneously 
for the remainder of the test period, 
with the replacement sensor connected to 



15 



the telemetry channel previously assigned 
to the SO2 analyzer. 

The oxygen detector was removed from 
the system after 30 days, owing to a 
steady decline in oxygen readings. The 
vendor discovered a faulty microfuel 
cell. Although the vendor's failure 
analysis was not conclusive, possible 
causes were an imperfection in the cell 
cathode, an inadvertent application of 
115-V-ac power to the cell during instal- 
lation, or a drop from a workbench to the 
floor when the unit was being packed for 
shipment to the test mine. Upon replace- 
ment of the faulty cell, the detector was 
returned to the system and operated with- 
out failure for the remainder of the 
test. 

During the 141-day test period, the 
temperature detector was the least active 
and the most stable. No adjustments or 
calibrations were required after the ini- 
tial installation and telemetry align- 
ment. Following its repair, the oxygen 
analyzer operated continuously for 75 
days with a stable and accurate trace. 
The original CO detector operated for 11 
days with an accurate and stable trace, 
and following vendor repairs, for the fi- 
nal 72 days of the test. 

Blasts were easily distinguishable on 
the chart recordings, showing peaks of 
30 to 39 ppm CO shortly after scheduled 
blast times, compared with background 
levels of to 6 ppm CO. During one 30- 
day period, 99 pet of the blasts that oc- 
curred were identified and confirmed. 
The replacement CO sensor tracked closely 
with the original unit when both were op- 
erating simultaneously. The replacement 
unit indicated abnormally high levels of 
CO during the peaks, however, with values 
ranging from 40 to 250 ppm. 

The SO2 sensor was very stable during 
the test period, with the only measurable 
recorder deflections attributed to slight 
telemetry alignment error. This perform- 
ance confirmed laboratory findings that 
SO2 production occurs only at higher tem- 
peratures and prompted the decision 
to terminate SO2 measurements after 64 
days so that its telemetry channel could 



be used for continued operation of the 
replacement CO sensor. The CO2 analyzer 
provided consistent, stable readings 
during the test period. Peak CO 2 mea- 
surements of 800 to 1,100 ppm correlated 
closely with blast patterns reported. 

Air samples were collected periodically 
during the period and analyzed in the 
laboratory. Close correlation between 
these measurements and the detection sys- 
tem was achieved. 

The five underground transmitters, five 
surface receivers, and power supply 
for the telemetry system performed well 
throughout the test. After initial in- 
stallation, the alignment was checked and 
adjusted twice. The purpose of checking 
and adjusting was more for familiariza- 
tion with the instruments than for cor- 
rection of drift. Surface receiver volt- 
ages were consistently identical with 
the input voltages from the underground 
transmitters. 

After the 5-month test period, a final 
functional test of the system was per- 
formed to evaluate system response to ab- 
normal levels of the sensed parameters. 
An induced spontaneous combustion event 
was staged in the vicinity of the detec- 
tion instruments to provide the required 
contaminants. A fuel source, consisting 
of 5 lb of pulverized mine timber mixed 
with 5 pet FeS and 1 pet S, was heated in 
an oven at about 390° F for approximately 
23 h. The oven was then opened and the 
fuel pile inspected. The pile center was 
heavily charred and flames ignited spon- 
taneously several times (figure 9). 

Chart paper traces of this test are 
shown in figure 10. Slightly elevated CO 
and CO2 levels were noticeable over the 
entire test period until about 16 h after 
heating began when both CO detectors in- 
dicated sharp increases. When the oven 
door was opened after 23 h, the CO level 
increased to about 27 ppm and the CO 2 
level showed a rise of about 250 ppm 
above ambient. Oxygen levels remained 
stable throughout the test at about 20.4 
pet, and no temperature change was 
observed. 



16 




FIGURE 9.— Induced spontaneous combustion test fire underground. 



Although this test was too short to al- 
low observation of the slowly increas- 
ing trend in gas and heat levels that are 
typical of a spontaneous heating event 
(the fast rate of contaminant rise ob- 
served when the oven door was opened is 
more indicative of a fire than incipient 
spontaneous combustion), it is signifi- 
cant that elevated CO and CO2 concentra- 
tions were detected and could be distin- 
guished from normal background levels of 
these gases. 



The failure of the SO2, oxygen, and 
temperature detectors to respond during 
the 5-month test or during the induced 
spontaneous combustion test indicates 
that these parameters, although poten- 
tially reliable indicators of fires in 
more advanced stages, may not be reliable 
indicators of incipient spontaneous com- 
bustion as it occurs in mines. 



SECOND-GENERATION SPONTANEOUS COMBUSTION FIRE DETECTION SYSTEM 



Operation of the first-generation spon- 
taneous combustion fire detection system 
was successful in that a capability for 
detecting the products of combustion an- 
ticipated during a spontaneous heating 
event was demonstrated. However, the 
5-month in-mine field test and final 
functional test also highlighted certain 
aspects of the system where improvements 
were needed. These needed improvements 
are summarized below: 



1. Actual oxygen and CO detector main- 
tenance frequency (monthly) was some- 
what more than anticipated. Although 
not out of line with manufacturer recom- 
mendations, it was in excess of the de- 
sign goal of 6-month maintenance-free 
operation. 

2. Analysis of detector output data 
was time consuming and confusing. Close, 
manual inspection of chart recordings was 
necessary to identify slowly developing 



17 



23 



— i 1 1 1 1 1 1 r 

CO Temperature C0 2 




S0 2 



Oven door_qpen_ ~21"i 



-f 



Recorder 
off 



CO, ppm 
C0 2 , ppm 
S0 2 , ppm 
2 ,pct 
Temp, "F 




10 15 20 25 30 35 40 45 50 



10,000 1,70 760 290 

10,000 1,7 00 760 290 

5 10 15 20 25 

15 30 45 60 75 90 105 120 135 150 



FIGURE 10.— Results of in-mine induced spontaneous com- 
bustion testing. 

problems (over several months, possibly). 
Confusion resulted because some traces 
indicated a positive deflection for in- 
creasing contaminant levels and others 
produced a negative deflection for in- 
creasing contaminant levels. Also, as 
airflows past the detection instruments 
changed, dilution of the sensed gases 
also changed, causing the detectors to 
report erroneously increasing or decreas- 
ing rates of contaminant generation. 

3. Three of the detectors, SO2, oxy- 
gen, and temperature, showed very little 
movement during both the 5-month test and 
the induced spontaneous combustion test. 



DESIGN MODIFICATIONS 

The second-generation spontaneous com- 
bustion fire detection system included 
three significant design modifications to 
Improve system performance. 

1. The detection instruments that re- 
quired the most maintenance (oxygen and 
CO) were either modified or eliminated 
from the system. Since oxygen did not 
prove to be a reliable spontaneous com- 
bustion indicator, oxygen detection was 
not included in the second-generation 
system. 

The CO detector's principal maintenance 
problems were the sample pump seizing and 
a grounding incompatability between a dc 
power circuit and the telemetry system. 
These problems were corrected by replac- 
ing the failed pump with a newer model 
featuring a modified mechanical linkage, 
and by bypassing the backup dc power 
operating mode. Following these modifi- 
cations, the detector operated without 
failure for the remainder of the test pe- 
riod. This sensor, as modified, was in- 
corporated into the second-generation de- 
tection system, along with two other 
electrochemical CO detectors that utilize 
diffusion-type cells. Use of diffusion- 
type cells eliminated the need for sample 
pumps. 

2. A more elaborate output display and 
system control was provided to simplify 
and enhance data analysis. The six-chan- 
nel chart recorder was considered a suf- 
ficient display for the 5-month test of 
the first-generation system. However, 
the difficulty and occasional confusion 
encountered in reading the charts indi- 
cated the need for several changes in 
output display, especially if longer time 
scales were anticipated. The new output 
display and system control provides audi- 
ble and visual alarm annunciation at in- 
dividually adjustable output levels for 
each detector, and more uniform, linear 
chart recorder traces with positive de- 
flections indicating increasing contami- 
nant levels. This control unit (fig. 11) 



18 




FIGURE 11.— Surface telemetry and control unit for second- 
generation spontaneous combustion system. 

was supplied by the manufacturer of the 
telemetry equipment and is intended for 
mine use. 

3. The detection instruments that 
showed little or no activity during the 
previous test were eliminated, and alter- 
native detectors were added to the sys- 
tem. As noted above, the oxygen detector 
failed during its first month of opera- 
tion; even when restored to proper func- 
tion, it showed little responsiveness for 
the remainder of the test (including the 



induced spontaneous combustion test). 
Its elimination from the system thus ne- 
gates a potential maintenance problem 
without adversely affecting system per- 
formance. The SO2 and temperature de- 
tectors were the most stable of all de- 
tectors tested, requiring almost no 
maintenance while maintaining proper cal- 
ibration over the entire 5-month test pe- 
riod. However, analysis of the results 
of the induced spontaneous combustion 
test indicated that measurement of the 
low level of SO2 and slight temperature 
rise expected from low-temperature incip- 
ient spontaneous combustion is not prac- 
tical from detectors placed in moderate 
to high airflows. Temperature and SO2 
would be better measured nearer to poten- 
tial hot spots and where ventilation is 
lower (i.e., in stopes, behind bulkheads, 
etc. ). 

Submicrometer particulate, or smoke, 
detection was included in the second- 
generation detection system. Although 
the laboratory pyrolytic decomposition 
studies indicated that smoke levels were 
less than the standard 2 pct/ft in the 
majority of tests, levels of 0.3 to 1.8 
pet were consistantly achieved, and lev- 
els exceeding 1.5 pet were measured in 
over half the tests. Using specially de- 
signed analog-output smoke detectors, 
these levels can be distinguished from 
normal background; thus, smoke may be 
considered a potential early indicator of 
spontaneous combustion. 

Two submicrometer particulate detectors 
were included in the system. The primary 
unit was supplied by the same vendor that 
manufactured the CO2 and SO2 analyzers 
and the telemetry and control system. 
The unit is a single-chamber, analog-out- 
put, ionization-type combustion particle 
detector. The ionizing source is 5 mCi 
of Kr-85 gas contained in a glass vial. 
The radioactive emitter ionizes the air 
in a chamber between two electrodes. 
Current is produced from production and 
transport of positive and negative ions 
to the opposite poles of the plates. A 
decrease in current, relative to clean 
air, is obtained when combustion products 
enter the chamber because the ionized 



19 



combustion particles are larger and heav- 
ier than the air molecules and move more 
slowly toward the end of the chamber. An 
electronic circuit detects the drop in 
current, which is proportional to the 
concentration of particles in the incom- 
ing air stream. The unit is intended for 
mine installation and is designed for 
long-term exposure to the harsh mine en- 
vironment. Previous experience with this 
detector under harsh mine conditions in- 
dicates excellent calibration stability 
with little maintenance required. 

The second submicrometer particulate 
detector, also an ionization type, is an 
experimental prototype model designed and 
fabricated by the Bureau. 

An air-velocity transducer was added to 
the system to permit more meaningful in- 
terpretation of reported contaminant lev- 
els. Changes in airflow that result in 
higher or lower smoke and gas concentra- 
tions can be factored into the data anal- 
ysis, thus avoiding erroneous conclusions 



3- pen chart recorders (3), 
surface console, 
annunciotor panel, 
event printer, 
telemetry receivers with 
alarm outputs (6) 



110 V — 


Combustion 

particle 

(Bureau 

experimental 

design) 


i 








no v — 


CO 

(diffusion — 
type) 












Combustion 
particle 





Telemetry 
transmitters (3) 



110 V- 



C0 2 



Telemetry 
transmitter 



CO 

(diffusion 
type) 



Combustion 
particle 



Spare Telemetry 

transmitters (3) 



.2 wires 
with ground 



Telemetry .. CO 



transmitters (3) 



i- Spare 



(pump type) 



Combustion 
particle 



Telemetry 
transmitter 



-110 V 



— CO, Telemetry 



Telemetry 
transmitters (3) 



CO 

(pump type) 



-110 V 



| Air velocity 



J Combustion 
particle 



Telemetry 
transmitter 



CO? 



FIGURE 12.— Layout of major components of second- 
generation spontaneous combustion fire detection system. 



regarding the source of contaminants. 
This unit is also intended for mine use 
and is designed for prolonged exposure 
to high temperatures, dust levels, and 
humidity. 

In addition to the design modifications 
of the system, the number of sampling 
locations was increased from one to four. 
This system expansion provided an oppor- 
tunity to evaluate a greater variety of 
detector types in various combinations 
and to test a larger capacity, more real- 
istically configured telemetry-control- 
alarm-display system. Layout of the 
second-generation spontaneous combustion 
fire detection system is shown in figure 
12. 

LABORATORY TESTS 

The entire system was thoroughly tested 
under controlled laboratory conditions. 
The primary objectives of the laboratory 
testing were to verify the compatibility 
of the various system elements and to 
identify potential in-mine installation, 
operation, maintenance, and troubleshoot- 
ing problems. Prior experience was con- 
sidered sufficient to justify omitting 
rigorous environmental exposure testing 
of individual components. No unusual 
problems were discovered during the labo- 
ratory testing, and in-mine installation 
and testing proceeded on schedule. 

IN-MINE TESTS 

In-mine testing was performed at the 
same Arizona copper mine where first- 
generation system testing occurred. Be- 
cause of project funding constraints, the 
installation was split into two phases. 
Sensor assemblies A and B, plus the nec- 
essary surface telemetry and control 
equipment, were installed first, followed 
by sensor assemblies C and D, 7 months 
later. Three of the four sensor assem- 
blies were located near the mine's No. 6 
exhaust shaft — each on a separate mining 
level. Sensor assembly A was located 
on the 3600 level, sensor assembly B on 
the 2550 level, and sensor assembly D on 
the 3400 level. Sensor assembly C was 



20 



located at the No. 4 exhaust shaft on the 
500 level. Figure 13 shows the locations 
of the sensor assemblies on a vertical 
section map of the mine. 

The sensors and telemetry modules for 
each underground assembly were mounted on 
a panel and prewired to simplify instal- 
lation (fig. 14). Minor difficulties 
were encountered during startup of assem- 
blies A and B, owing to telemetry mis- 
alignment resulting from transportation 
and handling of the control console. 
Several solid-state electronic components 
in the control console were also damaged 
during installation from transients on 
the transmission lines. Repairs were 
made by replacement with spare compo- 
nents, and normal system operation was 
achieved. 

Performance of sensor assemblies A and 
B during the initial 7-month test period 
was generally good. Day shift, afternoon 



shift, and night shift blasts were easily 
distinguishable on the chart recordings. 
Following blasts, typical CO levels were 
5 to 10 ppm and typical CO2 levels were 
450 to 650 ppm. The diffusion-type CO 
detector on the 3600 level tracked close- 
ly with the sample pump unit on the 2550 
level, with peaks occurring at corre- 
sponding times on the chart recordings. 
However, the levels of CO indicated by 
the diffusion-type sensor were a factor 
of 2 to 3 times higher than those of the 
pump-type unit. It is not clear whether 
these differences were real (actual CO 
levels that varied from area to area) or 
the result of instrumentation or teleme- 
try errors. 

Blasts were also clearly delineated on 
the smoke detector chart recordings; 
however, the actual particulate levels 
(percent obscuration, particles per cubic 
meter, etc. ) were uncertain because 



No. 7 


No. 5 


No. 8 


No. 3 


No. 4 


shaft 


shaft 


shaft 


shaft 


shaft 



No. 6 
shaft 




Sensor 
assembly B 



S .^Sprisnr 



Sensor—^ ^Sensor 

assembly A assembly D 



1,600 



Horizontal scale, ft 



FIGURE 13.— Section through test mine showing locations of sensor assemblies. 



21 




FIGURE 14.— Sensor assembly B, prewired and mounted on panel. 



calibration curves relating detector out- 
put to particulate levels were not avail- 
able from the detector's manufacturer. 

Maintenance frequency for the CO de- 
tectors was higher than desired. Each 
unit required zero adjustment and filter 
cleaning weekly, and the mechanical sam- 
ple pump was oiled about every 2 months. 
However the span calibration was per- 
formed only once during the 7-month peri- 
od, and that was during the week of in- 
stallation. Maintenance frequency for 
the CO2 and smoke detectors was lower 
than the design goal of twice per year. 
In fact, no maintenace, repair, or cali- 
bration was performed during the entire 
7-month initial test period. 

Intermittent telemetry failures and 
false alarms occurred as a result of 



electromagnetic interference with signal 
transmission. The telemetry lines used 
for transmission of detector outputs (ex- 
isting mine wiring was used for this 
purpose) were not shielded and were occa- 
sionally run in close proximity to high- 
voltage power cables. The interference 
was traced to radiation from these lines. 
Rerouting the telemetry cables a few feet 
away from the power lines corrected the 
problem, but its intermittent nature made 
troubleshooting a difficult and time-con- 
suming process. 

At the midpoint of the initial 7-month 
test period, a fire test was conducted to 
evaluate the performance of sensor assem- 
blies A and B. The site selected was on 
the 3600 level, about 650 ft upstream of 
sensor assembly A. Airflow through the 



22 



fire area was 30,400 ft 3 /min. This air 
combined with a 33, 100-f t 3 /min split up- 
stream of sensor assembly A. Total flow 
past sensor assembly B, consisting of the 
63,500 ft 3 /min from the 3600 level and 
75,500 ft 3 /min from the 3400, 3200, 3000, 
and 2550 levels, was 139,000 ft 3 /min. 
Combustion products from the fire were 
thus carried by the mine's ventilation 
past both sensor assemblies. Figure 15 
depicts the layout for the fire test. 

Permission to conduct the test was 
obtained from the U.S. Mine Safety and 
Health Administration, a rescue team was 
assembled, fire extinguishers and water 
were supplied to the test area, and as an 
added precaution, no personnel were per- 
mitted downstream of the fire. The fire 
was built in the bottom of a 55-gal drum 
that had been cut in half. The drum had 



holes along the bottom edge to draw air, 
and a screen was laid over the top as a 
spark arrestor. The primary combustible 
material consisted of 30 lb of 1-in scrap 
lumber. The wood was doused with char- 
coal lighter fluid and ignited (figs. 16- 
17). After 25 min, a piece of flexoid 
and 18 in of 1-in water hose was added. 
Ten minutes later, 6 oz of rock drill oil 
was added. After 65 min, all materials 
had been consumed and the residue was 
doused with water. 

Figures 18 and 19 show the chart traces 
recorded during the fire test. Table 3 
shows a comparison between CO and CO 2 
levels measured by the detection system, 
and results of a laboratory analysis of 
air samples collected near sensor assem- 
bly A on the 3600 level. Values at the 
2550 level are adjusted in the figures 




FIGURE 15.— Layout for In-mine fire test of second-generation spontaneous combustion fire detection system. 



23 



and table to reflect airflow differences 
(i.e., dilution) between sensor assem- 
blies A and B. 

All three traces show that the detec- 
tors on the 3600 level responded 3 to 5 
min earlier than the detectors on the 
2550 level (corresponding to the 1,100-ft 
separation between the sensor assem- 
blies). The measured gas concentrations 
show a slight variance from the labora- 
tory determinations; however, absolute 
agreement would not be expected under 
these test conditions (because of the 
possibilities of slight telemetry mis- 
alignment, imperfect mixing of air in the 
drifts, "non-plugged" flow, minor re- 
corder paper misalignment, ventilation 
uncertainty between the 3600 level and 
2550 level, etc.) 





FIGURE 17.— Test fire burning. 



40 



30 



20 



"i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — r 

A, CO 



Rubber hose 
added to fire 



3600 




J I I I I I I I I I I I I I I I L 




FIGURE 16. Igniting test fire. 



600 I I I I I I I I I I I I I I I I ' ' 

-25-20 -I5-I0 -5 5 10 15 20 25 30 35 40 45 50 55 60 65 

TIME, min 

FIGURE 18.— CO and C0 2 measured during fire test. 



24 



80 i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — i — r 



Rubber hose/ 
added to fire/ 



20 




J I I I I I I L 



J I I I I I L 



-25 -20 -15 -10 5 5 



10 15 20 25 30 35 40 45 50 55 60 65 

TIME, min 



FIGURE 19.— Smoke measured during fire test. 

The two smoke detector traces track 
closely, with the detector on the 3600 
level recording higher readings than the 
more distant detector on the 2550 level. 
As noted earlier, the diffusion-type CO 
sensor on the 3600 level recorded higher 
readings than its pump-type counterpart 
on the 2550 level. However, it is note- 
worthy that both the CO and C0 2 sensor 
pairs responded in similar proportions 
over the test period. E.g. , between 2 
and 15 min after the fire was ignited, 
both CO sensors increased by about 80 pet 
whereas both C0 2 sensors increased by 17 
pet. 



It is also noteworthy that the addi- 
tion of flexoid and hose to the fire are 
clearly distinguishable on the CO and 
smoke recordings. As these materials 
burn with a high emission of CO and 
smoke, but little C0 2 , a positive deflec- 
tion of the C0 2 trace was not expected. 

Following the initial 7-month test, 
sensor assemblies C and D plus the re- 
maining telemetry and control equipment 
were installed. Several faulty circuit 
boards were discovered in the new teleme- 
try panels when the system was energized, 
and the faulty boards were returned to 
the laboratory for repair. Before the 
circuit board repairs were completed, 
however, mine production was halted at 
the test mine due to depressed economic 
conditions and the low price of copper. 
The shutdown was indefinite, pending an 
improvement in the copper market, and it 
resulted in an almost immediate suspen- 
sion of activity related to the operation 
and maintenance of the detection system. 
Although project personnel eventually 
succeeded in completing the repairs to 
the system and achieving proper system 
operation, further testing of the system 
was impractical. Mine personnel re- 
quested that the equipment be retained at 
the mine site to supplement manual fire 
bossing; however, adequate staff to main- 
tain the system was not available. 

Project personnel visited the mine 
about 5 months after the shutdown to as- 
sess system operation under circumstances 
of almost no maintenance or calibra- 
tion. System operation was found to be 



TABLE 3. - Comparison of CO and C0 2 concentrations measured 
by detection system versus laboratory analysis 





Gas concentration, ppm 


Change in 




2 min after 


15 min after 


concentration, 1 




ignition 


ignition 


pet 


CO analysis: 










4 


7.2 


80 




12 


22 


83 




11 


31 


180 


C0 2 analysis: 










650 


760 


17 




655 


770 


18 




600 


700 


17 



1 Between 2 and 15 



min after ignition. 



marginal. The detection instruments at 
the one sensor assembly inspected (D on 
the 3,400 level) were only slightly out 



25 



of calibration, but telemetry problems 
resulted in inaccurate readings on the 
surface. 



SUMMARY AND CONCLUSIONS 



The potential seriousness of spontane- 
ous combustion fires in deep metal mines 
prompted the Bureau to perform research 
to upgrade technology for detecting such 
fires. The research was performed in 
three parts, beginning wi.th a laboratory 
study of induced spontaneous combustion 
events to identify and quantify reliable 
indicators of a spontaneous combustion 
fire in a metal mine. The recommended 
spontaneous combustion fire detection 
system included CO, CO2, oxygen, SO2, and 
temperature detection. 

The second stage of the program in- 
volved the assembly and evaluation of a 
prototype detection system. Tests were 
performed in the laboratory and in a deep 
metal mine with a history of spontaneous 
combustion problems. After 5 months un- 
derground, the system was functionally 
tested to evaluate response to abnormal 
contaminant levels. An induced spontane- 
ous combustion event was staged near the 
detection instruments to provide the re- 
quired contaminants. Of the five parame- 
ters monitored, only CO and CO2 were de- 
tected above background levels. The 
final stage of the program involved the 
design and long-term testing of a second- 
generation spontaneous combustion detec- 
tion system. Based on the previous stud- 
ies, CO and CO2 detection were included 
in the second-generation system; however, 
because of their relative inactivity dur- 
ing in-mine tests, the temperature, oxy- 
gen, and SO2 detectors were omitted. 
Submicrometer particulate (smoke) detec- 
tion was added, as the laboratory studies 
indicated that measurable smoke may be 
generated during a low-temperature spon- 
taneous combustion event. In-mine test- 
ing of the second-generation system in- 
cluded a staged fire test 3-1/2 months 
after installation. However, the test 
mine was forced to shut down, owing to 
economic conditions, before long-term 
endurance testing was completed. Two of 



the system's four sensor assemblies oper- 
ated for about 1 yr. The other two as- 
semblies received only a 3-week trial be- 
fore testing was halted. 

Overall, second-generation system per- 
formance was satisfactory, and installa- 
tion of similar systems in mines with a 
high risk of spontaneous combustion is 
recommended. The system demonstrated a 
capability to detect low levels of com- 
bustion products believed to be reliable 
indicators of spontanous combustion in 
metal mines. Principal problems and rec- 
ommended actions include (in descending 
order of significance) — 

1. The method of recording detector 
outputs (chart recorder) resulted in the 
generation of a large amount of data that 
needed to be manually analyzed for slow- 
ly developing trends. Computerized data 
storage and analysis is recommended to 
reduce labor requirements and possible 
data analysis errors. 

2. Both in-mine tests highlighted the 
need for controlling interference on te- 
lemetry lines. Shielded telemetry cables 
and placement away from sources of elec- 
tromagnetic interference are recommended. 

3. Although CO2 gas detection is rec- 
ommended, the nonlinear output of the CO 2 
detector used in this test program made 
data analysis difficult. Linearization 
of the CO2 output is recommended. If a 
computerized data storage and analysis 
system is used to record and analyze 
detector outputs, linearization of the 
CO2 unit's output could be incorporated 
into this system. 

4. The outputs of the smoke detectors 
used in this test program could not be 
correlated with commonly used quantita- 
tive measures of smoke exposure (obscura- 
tion, particles per unit volume, etc.). 
Standardized testing to develop an appro- 
priate calibration curve for this detec- 
tor is recommended. 



U.S. GOVERNMENT PRINTING OFFICE: 1 987 605-01 7 '60044 



INT.-BU.OF MINES,PGH.,PA. 28509 



244 



U.S. Department of the Interior 
Bureau of Mines— Prod, and Distr. 
Cochrans Mill Road 
P.O. Box 18070 
Pittsburgh. Pa. 15236 



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